Known combustion devices such as combustion devices that are part of a gas turbine are fed with a fuel and an oxidizer to generate flue gases that are expanded in a turbine.
In order to keep emissions (for example NOx, CO emissions) at low levels, premix combustion devices can be used.
In these kinds of combustion devices a fuel is first mixed with the oxidizer and the mixture is then burnt.
The fuel can be a liquid fuel (for example, oil) or a gaseous fuel (for example, natural gas or methane).
Gas turbines can be provided with supply lines for both liquid and gaseous fuel such that, if during operation one of the fuels is not available, operation can continue with the other fuel.
For these reasons it is often necessary to switch over from one fuel to the other. The switch over should be carried out online (e.g., during operation without stopping the gas turbine), because it is not possible to disconnect the generator driven by the gas turbine from the grid.
In order to switch over from one fuel to another fuel (both burnt in premix conditions) the amount of the first fuel is gradually reduced from its operational amount to zero, while, at the same time, the amount of the second fuel is gradually increased from zero to its operational amount. The total fuel supplied into the combustion device (e.g., being the sum of the first and second fuels) should be controlled such that it does not exceed a given amount that could cause damage.
Even if largely used, in some cases this switch over could generate pressure and thermal pulsations, which can be damaging and detrimental for the life of the combustion device. These conditions can also be detrimental for devices, such as, a turbine downstream of the combustion device and should therefore be counteracted for optimal performance.